Cucurbitacin B and Cucurbitacin C are the names of the compounds which cause bitterness in cucumbers. These compounds are found in all plant parts, but are concentrated in the stem end of the fruit. There is some in the blossom end, as well. Different varieties of cucumbers seem to contain more bitterness than others. You can cut off the ends to remove the bulk of the bitterness, but there will still be some under the skin of the cuke. Some people believe that if they peel a cucumber from the stem end to the blossom end, they'll remove more of the bitterness. There have been scientific studies, and the direction of peeling doesn't really make any difference in the amount of bitterness in a cucumber.

Cucumber and squash plants are susceptible to powdery mildew, generally towards the end of the season, when the nights are cold enough to stress the plants. In your gardening region I suspect the development of the disease has more to do with humidity and poor air circulation around the plants, than cool night air. You might try removing the older leaves, especially when they begin to show signs of distress. Give your plants as much direct sunshine as possible and increase the space between the plants to provide excellent air circulation.